There is a great need for a method which makes it possible to prepare elastomers whose level of impurities is so low, or can be influenced in a controlled way, that these impurities no longer have any effect on subsequent reactions and allow the elastomers to be used in the fields of medicine and contact with food.
Elastomers are widely used in many rubber-technology products, such as rubber tubing, transmission belts, seals, membranes, fabric coverings, shoe soles, profile sections, films, packaging materials and many other products. For applications in the field of injection-molded articles, applications involving contact with food, in the field of medicine, in the electronics industry and as starting products for further reactions, such as hydrogenations in the presence of sensitive transition metal catalysts, the elastomers need to have the impurities due to the elastomer production process, namely radical polymerization in an aqueous emulsion, removed from them in an elaborate way. The use of elastomers with too high an impurity level is therefore often greatly restricted on toxicological grounds in the medical sector and for contact with food, the limit on the impurity level being dependent both on the type of impurity and on its concentration. Impurity levels which are less than 2 wt. %, however, are preferred. The use of elastomers with too high an impurity level in electronic applications is often only conditionally possible. This is true especially when the impurities contain water and/or ions, since these can greatly affect the corrosion behavior and conductivity behavior of the electronic products, and they cannot always be removed by the action of heat without leaving a residue. Elastomers with an impurity level of more than 4 wt. %, expressed in terms of the purified elastomer, often cannot be used for reactions such as metatheses and/or hydrogenations, in which it is necessary to operate in the presence of sensitive transition metal catalysts, since the impurities make the reaction control more difficult, lengthen the reaction times and reduce the efficiency of the transition metal catalyst. In the case of hydrogenation, the impurities can furthermore contribute very significantly to corrosion and therefore wear of the systems needed for the hydrogenation. For many applications such as injection-molded articles or extruded articles, the use of elastomers with too high an impurity level (more than 3 wt. %) can lead to reduced surface quality of the articles, for instance mold contamination or efflorescence. Recovery of the impurities remaining in the elastomer is often of economic interest. This is true, in particular, when having used expensive catalysts which can be recycled after catalytic processing.
For the applications mentioned above, the purification of the elastomers with respect to impurities is typically carried out by expensive neutralization, coagulation, precipitation and washing processes in suitable organic substances, such as alcohols, ketones, ethers, water and mixtures thereof (Ullmann's Encyclopedia of Industrial Chemistry, Sixth Edition, 2000 Electronic Release), although complete purification cannot in principle be guaranteed. A particular problem involves the separation of low-molecular, sometimes high-boiling (>150° C.), weakly water-soluble and/or water-insoluble impurities such as emulsifiers, fatty acids, fatty acid salts and fatty acid esters, monomers and derivatives thereof, such as dimers, oligomers, undesired reaction products with starters and modifiers, modifiers/derivatives, organic catalyst residues and ligands, which remain in the elastomer phase when the elastomers have been prepared by emulsion polymerization and conventional purification processes. Using the processing and purification methods known in the prior art, these impurities often cannot be separated sufficiently, and even then only with significant economic outlay, since they become encapsulated during the latex coagulation of the elastomer and are therefore inaccessible to the washing processes. Fractional precipitation of the elastomers from solution, in order to separate low-molecular impurities and/or impurities soluble in organic solvents (precipitating agents) in which the polymer is insoluble, for example methanol as a solvent in the case of nitrile butadiene rubber and polychloroprene rubber, such impurities being for example emulsifiers, fatty acids, fatty acid esters, fatty acid salts of Na/K/Ca, monomers, modifiers, catalyst components, reaction products thereof, and subsequent drying, is known but entails high costs on the industrial scale. It is furthermore ecologically disadvantageous owing to the large amounts of solvents and precipitating agents required. It is therefore the object of the present invention to provide a method which makes it possible to reduce the impurity level of elastomers by up to 99 wt. % in relation to the unpurified elastomers.